Holograms that afford covert image(s) which are normally hidden and not viewable but which can be viewed in special circumstances and/or with special equipment are very useful in security applications, such as verifying authenticity of a product or a document.
Holographic imaging of a photosensitive film with reference and object beams being incident on the photosensitive film from air to produce a covert image that is normally hidden and scrambled is described in US Patent Publication 2002/0163678 and in GB 2317237A. The covert image is viewable as a recognizable image only with use of a suitable overlay plate, such as a lenticular plate. The hologram is improperly referenced and improperly reconstructed without inclusion of an overlay plate. Such improper reconstruction of the hologram without the overlay plate may equate to a fuzzy image that would lead one to believe that a security hologram is present.
The present invention includes a prism (or other element, e.g., a physical optic or a HOE) to reference a hologram. During reconstruction without the prism the image cannot be reconstructed, resulting in an invisible image. There is a need for a method utilizing an invisible security hologram until referencing occurs with the use of a prism. The present invention fulfills such need.